The invention relates to a color display device comprising a first substrate provided with a layer of fluorescent material, and a second, transparent substrate, said color display device comprising means for addressing pixels during use. The display device may be a plasma display device but also, for example, a display device of the field emission type. The address electrodes may be present on both the first and the second substrate in the case of a plasma display device. Dependent on the type of display device, the fluorescent material is patterned or not patterned.
A display device of this type is used, inter alia, in large-area, flat display screens, for example, for HDTV.
A color display device (plasma display panel or PDP) of the type mentioned above is described in EP-A-0 488 891. This application describes measures for obtaining grey values or color gradations. To this end, the frame time is divided into a plurality of sub-frames having a weighted duration (for example in the ratio 1:2:4: . . . :128). When a color, for example green, is fully driven, a frequently used phosphor such as Zn2SiO4:Mn (willemite) exhibits saturation phenomena. The efficiency of the phosphor (number of emitted photons/number of incoming photons) deteriorates, so that the brightness decreases.
It is, inter alia, an object of the present invention to provide a display device of the type described, which has an optimum brightness or luminance, also when a given color is fully driven.
To this end, a color display device according to the invention is characterized in that the layer of fluorescent material comprises at least two different phosphors of substantially the same color at the location of a pixel.
Substantially the same color is herein understood to mean that the difference in spectral color of the emission peaks of the phosphors, measured as the distance in C.I.E co-ordinates (x, y co-ordinates) is at most 0.35 (and preferably at most 0.25).
The invention is based on the recognition that the decrease of the efficiency for different phosphors depends in different manners on the number of incoming photons. By using two phosphors with a difference in a variation of the efficiency, it will be possible to mix colors of phosphors which, as regards color are (slightly) different but exhibit a different behavior as regards loss of efficiency in the case of full drive (maximum brightness), or to choose between the two phosphors.
An optimum effect is achieved when one of the two phosphors is a non-saturating phosphor. A non-saturating phosphor is herein understood to mean a phosphor in which the number of emitted photons per unit of surface and time has decreased at most 15% in the case of a drive yielding a brightness (or luminance) of 500 Cd/m2 as compared with a drive yielding a brightness of 10 Cd/m2. Upon excitation by means of an UV plasma which is maintained with alternating voltages, this means that, for example, at higher frequencies (at least up to 10 kHz and preferably up to 100 kHz or more) the efficiency (number of emitted photons/number of incoming photons) decreases by at most 15%. A frequently used phosphor such as Zn2SiO4:Mn (willemite) has already saturation phenomena from approximately 1 kHz in this application. The efficiency (number of emitted photons/number of incoming photons) has already dropped to approximately 90% and decreases rapidly at higher frequencies (to approximately 50% at 100 kHz).
However, for non-saturating phosphors, the efficiency remains substantially constant through a large frequency range.
Suitable non-saturating phosphors are, for example:
1) (Ce,Gd)MgB5O10:Tb, or CBT
2) (Ce,La,Gd)PO4:Tb or LAP
3) (Y,Gd)BO3:Tb.
A preferred embodiment of a display device according to the invention is characterized in that the layer of fluorescent material at the location of a pixel, viewed in a direction transverse to the first substrate, comprises at least two sub-pixels having different phosphors of substantially the same color for different sub-pixels. An incoming (video) signal can now be split up, for example, into two or more sub-signals each supplying one of the sub-pixels with a separate drive signal. By means of the sub-signals, the extent of mixing of the colors is then adjusted, or a choice is made between the sub-pixels.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiment(s) described hereinafter.